Its believed generally that when you collide two bodies with enough force, then the bodies group together by the attractive force and grow larger in size and mass..

Based on the similar concept, when two black holes collide, they just don't hit on each other, firstly, they get attracted by each others gravitational fields and they start they start to get close to each other, and hence due to that, they start to revolve around each other and finally their centers collide releasing a huge abount of energy :/ or some things like that I guess xD